I’m having longer-term issues with Wi-Fi

Use Wireless Diagnostics (enter its name in Spotlight, or hold Alt and click the Wi-Fi icon in the menu bar) to inspect your connection and compile a diagnostics report that you can read through yourself or send to Apple or another support agent – say, your ISP if you’re trying to get it to send a replacement router. If these tests don’t reveal immediate issues, opt to monitor your connection over time and watch for problems. Check out the options in the tool’s Window menu for other useful tools.